The proposed research addresses the central question of how muscles function under dynamic conditions of locomotor activity. It does so in the context of how muscle function is modulated in relation to muscle architecture and fiber composition to accommodate changes in locomotor requirement. These questions will be addressed by making in vivo recordings of force (tendon buckle transducers), length change (sonomicrometry) and neural activation (electromyography) of key limb muscles in two animal models: quadrupedal goats and bipedal guinea fowl. Measurements will be obtained from animals trained to move over a range of speeds on a treadmill at different gaits and grades (level vs incline vs decline) to address the following hypotheses: (i) regional activation and fractional length change within muscles that have focal skeletal attachments is uniform both along a fascicle axis and between differing fascicle regions, but may vary in muscles with broader attachments and more complex architectures; as a result, (ii) the timing and strain of activated fascicles are homogeneous within a muscle performing a given motor task; and (iii) proximal muscles with long fibers account for the majority of mechanical work modulation; whereas distal short-fibered muscles with long tendons contract isometrically for more economical force production and tendon elastic savings. Differences in mechanical work rate with locomotor grade will be related to observed changes in the in vivo force-length behavior of key limb muscles. Recordings made while animals accelerate from rest will provide a second context to evaluate work modulation in relation to muscle architecture. Ground reaction force-platform and high-speed video recordings will also be carried out to integrate the in vivo force, length and EMG measurements of individual muscles into whole-limb mechanics. These studies have important consequences for understanding patterns of motor recruitment in relation to locomotor strategy and how regional differences in motor unit organization (and fiber type) may influence the neural control of movement. Prior work in this area has been limited by studies of motor function under more quasi-steady ranges of movement and/or indirect assessment of muscle length change and force development. Although an overarching goal is to understand factors that influence normal and age-related changes in human motor function, animal studies allow direct experimental approaches for assessing the dynamics of motor function that are likely to apply to humans. Consequently, the proposed studies will have value for developing more effective physical, occupational and rehabilitative therapies, as well as for sports and exercise training, and prosthetics design.